Application of Vortex Methods to Coaxial Rotor Wake and Load Calculations in Hover

The coaxial rotor has distinct advantages in hover performance and forward flight compared to a conventional isolated rotor. The viscous vortex particle approach based on the Lagrangian formulation of the incompressible Navier-Stokes equations is a grid-free method suitable for capturing the vortex wake interaction over long distances. It is applied to evaluate performance, wake evolution, and loads in hover for a coaxial rotor system. A two-dimensional analysis is used to evaluate the effects of blade pitch angle and vertical separation distance. It is found that the interaction is significant only for distances of less than four to six chord lengths between the airfoils. In the coaxial rotor analysis, the performance is sensitive to the viscous drag coefficient of the airfoil. The rational function approximation of unsteady airfoil loads was used to reduce computational effort and improve accuracy of the calculations. The hover performance was predicted well with both the methods. Oscillations in thrust occurred primarily at blade passage azimuths. The rational function approximation calculations indicated a reduction in the magnitude of the thrust oscillations compared to the vortex panel method.